1. Field of the Invention
The present invention relates to a thermal line printer that prints an image on recording paper in a line sequential fashion, and more particularly to a thermal line printer that reduces unexpected density variations of the printed image caused by thermal deformations of the recording paper.
2. Background Arts
In a color thermosensitive line printer, a thermosensitive color recording paper having thermosensitive coloring layers for yellow, magenta and cyan is heated by a thermal head while being moved relative to the thermal head, to print a full-color image in a three-color frame sequential fashion.
As shown for instance in FIG. 12, a thermosensitive color recording paper 43 is moved past a thermal head 44 by conveyer roller pairs 42, each pair consisting of a capstan roller 40 and a nip roller 41. The thermal head 44 has a heating element array 46 consisting of a large number of heating elements 45 aligned along a main scan direction that is perpendicular to the conveying direction of the thermosensitive recording paper 43, as shown in FIG. 13. The thermal head 44 is driven to record one line of one color at a time on the thermosensitive recording paper 43, by applying variable heat energies from the heating elements 45 that are determined by coloring characteristics of the individual thermosensitive coloring layers as well as desirable densities or tonal levels of individual pixels designated by image data.
Since the heat energies are applied to the thermosensitive recording paper 43 while the heating elements 45 are pressed onto a surface of the thermosensitive recording paper 43, the surface of the thermosensitive recording paper 43 is thermally deformed. Especially at a print starting end 48 of an image recording area 47, that borders a blank margin 49, the temperature of the thermosensitive recording paper 43 steeply changes, so the deformation of the 10 is the largest. When the deformed portions of the thermosensitive recording paper 43 move past the conveyer roller pairs 42, conveying speed fluctuates because of the deformation of the thermosensitive recording paper 43.
As shown for instance in FIG. 14, if a convex 50 is formed at the print starting end 48 because of the thermal deformation, load on the conveyer roller pairs 42 is increased as the convex 50 moves past the conveyer roller pairs 42, so the conveying speed is lowered for that moment. As a result, the thermal head 44 applies more heat energies per unit area of the thermosensitive recording paper 43 than expected, increasing recording densities from designated levels in those lines which are recorded while the convex 50 is moving past the conveyer roller pairs 42. The lines having the increased recording densities appear as a stripe 51, as shown for example in FIG. 13. Such density unevenness, i.e. deviation from unexpected densities, always appears around a distance L1 from the print starting end 48, and the distance L1 is equal to a spacing between the thermal head 43 and the conveyer roller pairs 42.
The density unevenness or deviation provided in this way is inconspicuous if it is located in an area where a variety of colors and densities are mixed. However, the density deviation becomes so conspicuous if it appears in an area having similar colors or densities, like the sky, the sea or the facial area of a closed-up portrait, especially in an area having the same color and density, that the so-called black strip is apparent with the naked eye.
In view of the foregoing, an object of the present invention is to provide a thermal line printer and a printing method for the thermal line printer, by which the density deviation as caused by the thermal deformation of the recording paper is made less conspicuous, or reduced.
To achieve the above object in a thermal line printer having a pair of conveyer rollers for nipping and conveying a recording paper along a paper transport path, and a thermal head having an array of heating elements arranged transversely to the paper transport path and pressed onto the recording paper at a constant distance from a nipping position by the conveyer rollers, the thermal head being driven based on image data to record an image frame line by line from a print starting end of an image recording area on the recording paper as the recording paper is conveyed by the conveyer rollers in a direction advancing from the thermal head toward the conveyer rollers, the present invention suggests to provide an image analyzer for calculating a first estimation value representative of conspicuousness of potential density deviation the image frame could have in a first zone including a line that is to be recorded at the constant distance from the print starting end if the image frame is recorded from a top side thereof, and a second estimation value representative of conspicuousness of potential density deviation the image frame could have in a second zone including a line that is to be recorded at the constant distance from the print starting end if the image frame is recorded from a bottom side thereof. The image analyzer calculates the first and second estimation values based on image data of the first and second zones of the image frame respectively. Then, a control device compares the first and second estimation values to each other and controls the thermal head to start recording the image frame either from the top side or from the bottom side depending upon the estimation values such that the potential density deviation becomes less conspicuous.
The estimation values are preferably calculated based on histograms each showing the numbers of pixels contained in each of the zones in relation to tonal levels of the image data, and correction coefficients for correcting the histograms to make the estimation values the larger the more the potential density deviation becomes conspicuous. The numbers of pixels shown in relation to the tonal levels may be called pixel frequencies, and correction coefficients may be called weighting coefficient.
Where the recording paper is a color thermosensitive recording paper having at least three coloring layers for yellow, magenta and cyan, each of the estimation values is calculated by obtaining a density histogram for each color, correcting the histogram with a first kind of correction coefficients determined for each color and with a second kind of correction coefficients that weight the number of those pixels included in a middle density range more than other density ranges, and adding up the histograms for the three colors.
It is preferable to calculate a third estimation value representative of conspicuousness of potential density deviation that the image frame could have in a third zone including a line that is to be recorded at the constant distance from the print starting end if the image frame is recorded from a right side thereof, based on image data of the third zone, and a fourth estimation value representative of conspicuousness of potential density deviation that the image frame could have in a fourth zone including a line that is to be recorded at the constant distance from the print starting end if the image frame is recorded from a left side thereof, based on image data of the fourth zone. The first to fourth estimation values are compared to one another, to decide to start recording the image frame from one of the four sides which corresponds to the smallest estimation value.
According to another aspect of the present invention, a printing method for a thermal line printer wherein an array of heating elements of a thermal head are pressed onto a recording paper and driven in accordance with image data to record an image frame in an image recording area on the recording paper line by line from a print starting end of the image recording area, as the recording paper is conveyed by a pair of conveyer rollers in a direction advancing from the thermal head toward the conveyer roller pair, the conveyer rollers nipping the recording paper at a constant distance from the pressing position of the heating element array, comprises the steps of: picking up image data of a first line that is to be recorded at the print starting end; calculating based on tonal levels of the picked up image data an amount of heat energy applied from the thermal head to the recording paper at the print starting end; deriving from the heat energy amount a deformation amount of the recording paper; calculating based on the deformation amount a density correction value; and correcting recording densities of a line located at the constant distance from the print starting end, in accordance with the density correction value.
Because a plurality of lines located around the constant distance from the print starting end several lines can suffer the density deviation, it is preferable to obtain a first density correction value from image data of the first and following several lines, and correct recording densities of the plurality of lines around the constant distance, in accordance with correction values that are determined for the respective lines based the first density correction value.